RPI safe mode
This results in config.txt being ignored (except for avoid_safe_mode). A default cmdline.txt is applied, and kernel_emergency.img is loaded (if not available, then just kernel.img).
It provides a means of recovering from, e.g.
- too high an overclock
- a typo in cmdline.txt
- an incorrect kernel=<filename> parameter
- an invalid display mode
without requiring a PC. (Obviously it won't help if you've deleted start.elf...)
The busybox based kernel_emergency.img has /boot mounted, and vi installed so you can fix up config.txt or cmdline.txt. (You can also mount /dev/mmcblk0p2 and fix up the rootfs if you have problems there that stop you booting).
Connect pins 5 & 6 with a jumper or piece of wire. These pins are the 3rd row from the end of the P1 header closest to the edge of the board.
Be very careful with where you insert the jumper. Some pins carry power, so shorting those would not be a good idea. Wire insulation stripped off of 20 AWG wire can be used to cover the power pins and help prevent accidents.
P1 Pin diagram: RPi Low-level peripherals